The present invention relates generally to a control device for reducing premature brake wear in mobile self propelled machines. In particular, the present invention is a brake actuated shift control for machines having a transmission with at least two ranges.
Mobile machines or vehicles, such as mobile trenchers, skid steer loaders, trucks, tractors and the like utilize braking systems to provide a braking action to restrict movement of the mobile machine. The braking action can be initiated by a variety of methods, including depressing a pedal, actuating an electrical switch, or moving a brake engagement lever. A conventional hydraulic brake system typically includes a brake actuation valve coupled to a brake actuation piston. The brake actuation piston moves a brake pad or brake shoe which in turn engages a brake disk or brake drum, respectively, coupled to a drive driving a wheel. The braking system generally includes a parking brake which, when engaged, with drive power disengaged, allows an operator to leave the mobile machine unattended.
An operator occasionally causes brake failure by "driving through the brake", or moving the machine during such time that the brake is engaged, if the braking force is insufficient to hold the brake drum or disc stationary with drive power engaged.
"Brake drive through" is most prevalent in situations where the transmission produces a high output torque and substantially overpowers the braking force such that the braking force is unnoticeable to the operator. Generally, the output torque applied by the transmission corresponds inversely to the drive range selected. For example, the lowest range available in the transmission would correspond to the highest output torque available by the transmission. This is also true when the vehicle has a multiple range fluid actuated transmission such as, for example, a hydrostatic transmission.
A hydrostatic transmission includes a pump section which is driven by an engine of the vehicle to transmit hydraulic fluid under pressure through hydraulic circuitry to a motor section of the hydrostatic transmission. The motor section drives the vehicle wheels. The ground speed of the machine is determined by the volume of oil supplied to the motor section within a given period of time.
Selectable ranges or speeds in hydrostatic transmissions allow the operator to select an appropriate maximum speed for a particular application. A range which provides for a higher maximum speed than another range results in an overall reduction of available output torque in a manner analogous to the speed/torque characteristics of a conventional manual transmission. For example, fourth gear in a four speed manual transmission provides the fastest vehicle speeds, however, the same vehicle cannot easily be set in motion when the transmission is in fourth gear due to the reduced available output torque.
Brake actuated indicator lamps, bells, buzzers or the like, are used as warnings to reduce brake drive through. However, such an indicator is only partially successful in reducing premature brake wear since the lamp can burn out, the view of the lamp may be obscured by dust or glare from the sun, or the operator may disregard the indicator lamp entirely. The possibility of "brake drive through" can be further reduced by shifting the transmission into neutral so that the transmission does not supply an output torque to a drive wheel.